This invention relates to the polymerization of 2-pyrrolidone and more particularly to the formation of polymers which have a useful molecular weight and are white in color when using sulfur dioxide as the chain initiator.
The formation of polymers of 2-pyrrolidone involving the use of alkaline catalysts via an anionic mechanism is disclosed in U.S. Pat. No. 2,638,463. Subsequent patents, for example U.S. Pat. No. 2,809,958, further disclose the need for an activator or co-catalyst to increase the yield of polymer formed.
Suitable alkaline catalysts are the oxides, hydroxides, alcoholates, hydrides, amides, etc. of the alkali metals, as well as the alkali metals themselves, which form the alkali metal salt of 2-pyrrolidone ##STR1## which is the actual catalyst.
The simplest, and most free from side reactions, are the alkali metal hydroxides, particularly sodium or potassium hydroxide. Quaternary ammonium hydroxides, as disclosed in U.S. Pat. No. 2,973,343, are also useful provided care is taken not to overheat the mixture during the preparation of the pyrrolidone salt which would destroy the thermally unstable quaternary base.
The polymer formed is believed to be a linear polyamide which has been called polypyrrolidone or nylon-4, having the structure: ##STR2##
The value of n, or in other words, the molecular weight of the polymer, has been found to be dependent on the type of activator employed. The vast majority of activators or co-catalysts which have been disclosed in the prior art (for example those disclosed in U.S. Pat. Nos. 2,809,958; 3,060,153; and 3,069,392 result in a polymer of medium molecular weight having insufficient thermal stability to withstand the high temperatures required for melt extrusion processes, for example in the formation of textile fibers by melt spinning.
Only two of the numerous activators which have been disclosed are capable of forming polymers of 2-pyrrolidone which have markedly higher molecular weights and greatly increased thermal stabilities. One of these is carbon dioxide, disclosed in U.S. Pat. No. 3,721,652 and the other is sulfur dioxide, disclosed in U.S. Pat. No. 3,174,951. Polymers formed by using both CO.sub.2 and SO.sub.2 activators have been successfully melt spun into fibers (Gunter Schirawski, Die Makromolekulare Chemie, 161 (1972), page 67).
There is, however, one drawback to the polymers formed by the use of SO.sub.2 as the chain initiator; they are either quite yellow or have a yellowish tint when made under conditions which have been regarded as optimum for yield and molecular weight. These yellow polymers are much less useful commercially than the white polymers formed via the CO.sub.2 route.
In the original disclosure of SO.sub.2 as a chain initiator in the polymerization of 2-pyrrolidone (example 1 of U.S. Pat. No. 3,174,951) only one concentration of SO.sub.2 was tried and although this gave a polymer having a high molecular weight, it was obtained in a yield of only 13.6 o/o. Attempts to improve the yield by employing the customary 50.degree. C polymerization temperature results in a product having a deep golden yellow color and a viscosity so low as to be useless for extrusion purposes of any kind.
Jarovitzky (U.S. Pat. No. 3,681,294) added a mixture of sodium sulfite and sodium bisulfite to the polymerization mixture in an effort to improve the yield without forming a yellow product. During the course of his work he also carried out some experiments without any additives. In his Table I he reports his findings as to color for both the wet and the dry polymers. We have found that differences in color are much more readily discernable in the wet polymer than in the dry polymer. For example, in Example 2a of U.S. Pat. No. 3,681,294 the wet, room temperature polymer is described as "light yellow" and the wet 50.degree. C polymer as being "orange yellow", while both of these polymers are described as "white" when dry. The surface of the dried polymer is dull and scatters the light more and unless comparisons are made with a standard color reference, even quite yellow polymers may be classified as "white".
In our work we used a polymer prepared via CO.sub.2 activation as a standard reference sample for determining the whiteness of the polymers made in Examples 1-38 (Table I). Polymers of 2-pyrrolidone made via CO.sub.2 activation set a new standard for whiteness for nylon-4, being much whiter than those made using other activators and fully comparable with polyester in this respect.
Each sample in Examples 1-38 was compared side by side with the polymer made by the CO.sub.2 route, both wet and dry. The wet samples only were reported since we found the color of the wet samples to be more discernable than the dry as mentioned above. Prior art observations of the whiteness of nylon-4 samples have been rather superficial in that no standard reference sample was employed. Visual comparisons side by side with a reference sample are very critical and even very slight differences in color can be readily detected even though it is not possible to assign a numerical value by this method. Great care was exercised by both of us in our whiteness determinations.
Of the examples in U.S. Pat. No. 3,681,294 without any additives, only one, namely, Example 4a in Table I, was classified as "white" when wet. In this example there was employed an SO.sub.2 concentration of 0.0017 mols per mol of 2-pyrrolidone, but the yield of product even at a polymerization temperature of 50.degree. C was only 4 o/o and the intrinsic viscosity of 1.7 was so low that the product could not be used in any melt extrusion process.
Schirawski, in the publication cited above, also studied the effect of various concentrations of SO.sub.2 on the yield, viscosity and thermal stability of polymers formed by the use of SO.sub.2 as the activator, without regard however, to the formation of colored polymer. In Table 7 on page 65 of his publication he finds that an SO.sub.2 concentration of 0.017 mol per mol of 2-pyrrolidone gives both a maximum yield and a maximum viscosity but a poor thermal stability. Although he does not mention the color of the polymer formed in this experiment specifically, we have found that this concentration of SO.sub.2 at 50.degree. C does give a polymer which is yellow in color. On page 66 of his publication it is stated that the higher concentrations of SO.sub.2 resulted in yellow-brown to yellow products.
It is an object of this invention to provide polymers of 2-pyrrolidone made by the use of SO.sub.2 as the activator or chain initiator which are fully equivalent or superior to those made by the use of CO.sub.2 in all respects including viscosity, color and thermal stability.
It is a further object of this invention to provide a method of controlling the viscosity of the polymeric product formed within critical limits.
It is a still further object of this invention to provide polymers of 2-pyrrolidone made by the use of SO.sub.2 as the initiator which are as good as or better than those made via CO.sub.2 initiation in all respects without the incorporation of any other active ingredients.
Other objects will be apparent in the detailed disclosure which follows.